chore: add haarchri to maintainers in OWNERS.md

[jbw976]

Description of your changes

Follow-up to crossplane/crossplane#7069 to add @haarchri as a maintainer on crosspplane-runtime too.

I have:

• Read and followed Crossplane's contribution process.
• Run earthly +reviewable to ensure this PR is ready for review.
• Added or updated unit tests.
• Linked a PR or a docs tracking issue to document this change.
• Added backport release-x.y labels to auto-backport this PR.

Need help with this checklist? See the cheat sheet.

[coderabbitai]

📝 Walkthrough

Walkthrough

A new maintainer entry for Christopher Haar is added to the OWNERS.md file, expanding the Maintainers section with a public-facing record including their email and GitHub handle.

Changes

Cohort / File(s)	Summary
Maintainer Entry Addition OWNERS.md	Added Christopher Haar as a new maintainer with contact information (christopher.haar@upbound.io) and GitHub handle (haarchri).

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Possibly related PRs

• chore: update OWNERS.md with Crossplane maintainer team #903: Also modifies the OWNERS.md maintainer list, directly related maintainer management changes.

Suggested reviewers

• negz
• bassam
• bobh66
• phisco

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and descriptively summarizes the main change: adding a new maintainer to the OWNERS.md file, staying well under the 72-character limit.
Description check	✅ Passed	The description is directly related to the changeset, explaining the purpose as a follow-up to add @haarchri as a maintainer, with appropriate context and completed checklist.
Breaking Changes	✅ Passed	PR only modifies OWNERS.md administrative documentation with no changes to any Go source files, so public API cannot be affected.

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